Queen Elizabeth II (when she was still called Princess Elizabeth) was heiress presumptive to her father King George VI, because she was his oldest daughter and he had no sons. She was not his heir apparent, though, because if the king ever had a son he would have taken over as heir apparent.

↑Richard Burn; John Burn, A New Law Dictionary (London: printed by A. Strahan and W. Woodfall, for T. Cadell, 1792), p. 423

↑The Manual of Rank and Nobility, or Key to the Peerage (London: Saunders and Otley, 1828), p. 43

↑Jeremy Paxman, On Royalty: A Very Polite Inquiry Into Some Strangely Related Families (New York : PublicAffairs, 2008), p. 103

↑John Almon; John Debrett; et al., The Parliamentary register: or, History of the proceedings and debates of the House of Lords ... during the 14th-[18th] Parliament[s] of Great Britain [1774-1803] (London: Printed for J. Almon, 1775-1804), p. 75